7. Lambda With Receiver [kotlin-workshop]

Part of https://github.com/jetBrains/kotlin-workshop.

Covers:
- the concept of lambda with receiver
- DSLs examples
- with/apply/also/let/run functions

Transcript

1. Lambda with receiver
2. Extension Function & Lambda Lambda with receiver
3. val sb = StringBuilder()
 with (sb) {
 appendln("Alphabet: ")
 for (c in 'a'..'z') {
 append(c)
 } toString()
 } The with function with is a function val sb = StringBuilder()
 sb.appendln("Alphabet: ")
 for (c in 'a'..'z') {
 sb.append(c)
 }
 sb.toString()

4. lambda is its second argument val sb = StringBuilder()
 with (sb) {
 this.appendln(“Alphabet: ")
 for (c in 'a'..'z') {
 this.append(c)
 } this.toString()
 } val sb = StringBuilder()
 with (sb, { ->
 this.appendln(“Alphabet: ")
 for (c in 'a'..'z') {
 this.append(c)
 } this.toString()
 }) lambda is its second argument Lambda with receiver with is a function this is an implicit receiver in the lambda val sb = StringBuilder()
 with (sb) {
 appendln("Alphabet: ")
 for (c in 'a'..'z') {
 append(c)
 } toString()
 } this can be omitted
5. Lambda with receiver val sb = StringBuilder()
 with (sb) {
 appendln("Alphabet: ")
 for (c in 'a'..'z') {
 this.append(c)
 } } lambda with implicit this
6. regular function regular lambda extension function lambda with receiver Extension function vs lambda with receiver
7. regular function regular lambda extension function lambda with receiver fun String.lastChar() = this.get(this.length - 1) buildString { this.append("...") } Extension function vs lambda with receiver
8. regular function regular lambda extension function lambda with receiver Lambda vs lambda with receiver val isEven: (Int) -> Boolean = { it % 2 == 0 } val isOdd: Int.() -> Boolean = { this % 2 == 1 } isEven(0) 1.isOdd() calling as regular function calling as extension function
9. val s: String = buildString {
 appendln("Alphabet: ")
 for (c in 'a'..'z') {
 append(c)
 }
 } Another example: buildString lambda with receiver
10. buildString { this.append("...") } inline fun buildString( builderAction: StringBuilder.() -> Unit ): String { val stringBuilder = StringBuilder() ... return stringBuilder.toString() } The buildString function Creates a StringBuilder Calls the specified actions on a stringBuilder Returns String as a result stringBuilder.builderAction()
11. with (sb) {
 appendln("Alphabet: ")
 ...
 } inline fun <T, R> with( receiver: T, block: T.() -> R ): R = receiver.block() The with function declaration
12. val db: SQLiteDatabase = … db.beginTransaction()
 try {
 db.delete("users", "first_name = ?", arrayOf("Jake"))
 db.setTransactionSuccessful()
 } finally {
 db.endTransaction()
 } db.inTransaction {
 delete("users", "first_name = ?", arrayOf("Jake"))
 } Avoiding Duplication
13. db.beginTransaction()
 try {
 db.delete("users", "first_name = ?", arrayOf("Jake"))
 db.setTransactionSuccessful()
 } finally {
 db.endTransaction()
 } db.inTransaction {
 delete("users", "first_name = ?", arrayOf("Jake"))
 } Inline functions is declared as inline function generated bytecode is similar to
14. html {
 table {
 for (product in products) {
 tr {
 td { text(product.description) }
 td { text(product.price) }
 td { text(product.popularity) }
 }
 }
 }
 } HTML Builders lambdas with receiver
15. Anko Layouts DSL for dynamic layouts
16. Alerts fun Activity.showAreYouSureAlert(process: () -> Unit) {
 alert(title = "Are you sure?",
 message = "Are you really sure?") {
 positiveButton("Yes") { process() }
 negativeButton("No") { }
 }.show()
 } Are you sure? Are you really sure? No Yes
17. customView {
 verticalLayout {
 val email = editText {
 hint = "Email"
 }
 
 val password = editText {
 hint = "Password"
 transformationMethod = PasswordTransformationMethod.getInstance()
 }
 
 positiveButton("Log In") {
 logIn(email.text, password.text)
 }
 }
 } Password Log In Email Custom layouts
18. plugins { application kotlin("jvm") version "1.1.51" } application { mainClassName = "samples.HelloWorldKt" } dependencies { compile(kotlin("stdlib")) } repositories { jcenter() } Gradle Build Script in Kotlin
19. More useful library functions inline fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block() inline fun <T, R> T.run(block: T.() -> R): R = block() inline fun <T, R> T.let(block: (T) -> R): R = block(this) inline fun <T> T.apply(block: T.() -> Unit): T { block(); return this } inline fun <T> T.also(block: (T) -> Unit): T { block(this); return this }
20. with with (window) { width = 300 height = 200 isVisible = true } inline fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()
21. run: like with, but extension val windowOrNull = windowById["main"] windowOrNull?.run { width = 300 height = 200 isVisible = true } inline fun <T, R> T.run(block: T.() -> R): R = block()
22. run: like with, but extension windowById["main"]?.run { width = 300 height = 200 isVisible = true } inline fun <T, R> T.run(block: T.() -> R): R = block()
23. apply: returns receiver as a result val mainWindow = windowById["main"]?.apply { width = 300 height = 200 isVisible = true } ?: return inline fun <T> T.apply(block: T.() -> Unit): T { this.block(); return this }
24. also: regular argument instead of this inline fun <T> T.also(block: (T) -> Unit): T { block(this); return this } windowById["main"]?.apply { width = 300 height = 200 isVisible = true }?.also { showWindow(it) }
25. { .. this .. } { .. it .. } return result of lambda run let return receiver apply also receiver.apply { this.actions() } receiver.also { moreActions(it) }
26. Copyright © 2017 https://github.com/JetBrains/kotlin-workshop